DOCS. 169-170 DECEMBER 1915 165 Mercury’s perihelion and approximately solved. It is easy to specify the most general line element which has the necessary symmetry properties. Proceeding from this, I initially determined the first-order approximation in your way and found: 9pa = -axpxo/r3 - ßxpxo/r3 = Spa [B/3r3] g44 = 1, therefore, two arbitrary quantities a and ß, which was peculiar. Then I moved to the solution as a whole.” 170. To Hans Albert Einstein Berlin, 23 December 1915 My dear Albert, I have been working so hard in the last few months that I urgently need a rest during the Christmas holidays. Aside from this, coming across the border is very uncertain at present, since it has been almost constantly closed recently.[1] That is why I must unfortunately deprive myself of visiting you now. But instead I’m definitely coming in the first days of April. Then I have more time and can wait at the border until I am allowed into Switzerland. I have sent my Christmas gift to you both in the form of money, according to your wish. I’m glad that you romp around a lot out of doors. I hope there will continue to be a decent amount of snow as well so that you can do your skiing. Ask Mama to send me a photo of you both again. You have grown again and have changed since the last one. Write me about the things you are up to besides school, and who your friends are. Is there a subject in or outside of school that interests you particularly? Then I would also like to hear from you what schooling you have ahead of you, because I am not exactly familiar with the Zurich school system.[2] How is Tete’s health? Does he ever have complaints? Does he look well? Now to something else of importance. I have heard that there is an excellent remedy for tooth and bone development especially for growing children, which you both should take. You prepare a saturated solution of calcium chloride (CaCl2) and store it in a corked medicine bottle. After every meal you put one little teaspoonful of it in half a glass of water (or milk) and drink it. It would be a true blessing for Tete if he were to take it. You see, calcium (Ca) is a metal that plays a major role in teeth and bones. It is present in a digestible form in natural food, but during cooking it precipitates out in an indigestible form so the body cannot benefit from it any more. Talk also to Mama about this; she will immediately see the sense in it. Write to me again soon, my dear Albert, and on replying have my letter before you and answer everything I have asked you. In this way it’s also easier for you